The present invention relates to semiconductor integrated circuits and, more particularly, to an output buffer which is biased by a voltage regulator for driving voltages that are greater than transistor tolerances.
CMOS integrated circuits are typically provided with tri-state output drivers that are selectively operable between a normal drive mode and a tri-state or high impedance mode in which the drivers appear transparent to the output terminals to which they are connected. Advancements in semiconductor integrated circuit fabrication technology enable the geometries of semiconductor devices to be progressively reduced so that more devices can fit on a single integrated circuit. As a result, core voltages of the integrated circuits are being reduced to prevent damage to the small devices and to reduce overall power consumption. For example, power supplies are now being reduced from 5V to 3.3V, and from 3.3V to 2.5V.
However, low voltage CMOS devices are often interconnected at a board level to TTL logic and other devices that operate at higher supply voltages of 5V or 3.3V. If no precautions are taken, an external 3.3V level signal applied to the output terminal of a 2.5V tri-state driver can cause voltage drops across the transistor devices in the driver that exceed the transistor tolerances, which can cause the gate oxide of the devices to break down. It is therefore desirable to provide an output driver that can drive large voltages off of the integrated circuit without exceeding the tolerance levels of the devices within the driver.